LibCore: Remove unused EventLoop::is_main_event_loop()

Author: https://github.com/gmta
Commit: 9e2a619fdc
Pull-request: https://github.com/SerenityOS/serenity/pull/13784
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibCore/EventLoop.cpp

@@ -425,11 +425,6 @@ public:
     }
 
 private:
-    bool is_main_event_loop()
-    {
-        return s_main_event_loop.with_locked([this](auto* main_event_loop) { return &m_event_loop == main_event_loop; });
-    }
-
     EventLoop& m_event_loop;
 };